Light emitting diode (LED) has commonly been used in many fields for different purposes, such as lightening, signaling and displaying. Although LED industry made a great leap during last two decades, uniformity of light and lightening efficiency are still two aspects that are not easily controlled by current manufacturing process. This is because attachment of phosphors is done by gluing. It causes non-uniform phosphor distribution in the glue or an irregular gluing body. A common defect of such a LED shows annular yellow rings.
Please refer to FIG. 1 (Prior Art). A conventional LED package 10 is shown. The LED package 10 has first and second lead frames 12 and 14, by which electrical power is supplied to the LED package 10. The lead frame 12 has a recessed reflector area 16 in which is disposed an LED 18. The LED 18 is made from an indium-doped gallium nitride epitaxial layer on a transparent sapphire substrate. When activated by a direct current at an appropriate forward voltage, the top surface of the LED 18 of indium gallium nitride produces a blue light at approximately 470 nm wavelength.
The LED 18 is connected by a wire bond 20 to the lead frame 12 and by a wire bond 22 to the lead frame 14. The LED 18 has a layer of fluorescent material 24 disposed over it. The fluorescent material 24 is generally a transparent epoxy resin containing particles of YAG/Gd:Ce phosphors. The entire assembly is embedded in a transparent encapsulation epoxy resin 26.
Also shown in FIG. 1 (Prior Art) are arrows 28 and 30, which represent the light rays of an annular blue ring. The arrows 32 and 34 represent the light rays of an outer annular ring, and the arrows 36 and 38 represent an inner annular yellow ring.
Referring to FIG. 2 (Prior Art), the lead frame 12 is shown with its reflector portion 16 which forms a cup holding the LED 18. Shown closer up is the layer of fluorescent material 24 having thin areas 40 and 42 and a thicker area 44. The final encapsulation epoxy resin 26 is not shown for purposes of simplicity.
Where the layer of fluorescent material was relatively thin at areas 40 and 42, shown in FIG. 2, the blue light would generally provide a blue annular ring along the light ray lines 28 and 30 since there would be insufficient contribution of light from the phosphors. Inside and out of the annular blue ring would be yellow annular rings due to light rays 32 and 34 and light rays 36 and 38 where the phosphors would contribute some light but not enough to create a uniform white light. It has been determined that the surface tension of the material 24 over the LED 18 causes areas of various thickness which range from the thicknesses at areas 40 and 42 by the corners of the LED 18 and the thickness at area 44 above the center of the LED. This causes non-uniform reradiation of the blue light and the annular rings previously described. Here, we can see the problem of non-uniformity of phosphor layer.
In order to solve the aforementioned problem, U.S. Pat. No. 5,959,316 disclosed an improving structure for LED encapsulation. In the invention as shown in FIG. 3, a surface-mounted LED light 52 disposed on a device substrate 54 of a surface mount device. The LED 52 is encapsulated in a transparent spacer 56 which is further covered by a layer of fluorescent material 58 and a final transparent encapsulation layer 50. It is possible to utilize surface tension (which at the size of an LED 52 is large relative to gravitational forces) in combination with viscosity to allow the drop of a hemispherical shape of a viscous, transparent ultraviolet (UV) light-cured resin over the LED 52 which forms the transparent spacer 56. The resin would cover all the corners and then be cured by using UV light. This would then be followed with the layer of the fluorescent material 58, a viscous UV cured resin. The deposition of the transparent spacer 56 would provide a hemisphere. Then the layer of fluorescent material 58 would flow to conform to the hemispherical shape of the transparent spacer 56 and be cured prior to the final encapsulation 58 are formed. Although the invention would not be easily subject to the annular ring problem, manufacturing process for forming the layer of fluorescent material 58 in practice often encounters non-uniform distribution of phosphors in the mixture of fluorescent material 58. Shape of the transparent spacer 56 is not perfectly hemispherical. It is still difficult to provide a good yield rate of products.
Another invention described in U.S. Pat. No. 7,278,756 provides an innovative way to improve the problem of uniformity of light. Please refer to FIG. 4. It is a schematic cross-sectional view of the LED in accordance with the '756. The LED 60 comprises a chip body 68 for emitting light, an encapsulation can 66 surrounding the chip body 68 and having a light emitting surface 62, and a base 69 supporting the encapsulation can 66 and the chip body 68. The encapsulation can 66 has numerous fluorescent particles 64 arranged there.
The fluorescent particles 64 are distributed in a region adjacent to the light emitting surface 62, distal from the chip body 68. The fluorescent particles 64 progressively increase in size with increasing distance away from a center axis of the region. The fluorescent particles 64 scatter light emitted from the chip body 68 to improve luminance and uniformity of illumination. However, in practice, it is also a challenge to achieve such particle arrangement.
Recently, U.S. Pat. No. 7,479,662 provides a method to overcome the defects of LED mentioned above. As illustrated in FIG. 5, an LED package 70 includes an LED chip 72 mounted on a substrate 74, which in turn is mounted on a reflector 76. A lens 78 surrounds the chip 72, the substrate 74 and reflector 76. Optionally filling space 82 between the lens 78 and the chip 72 is typically an epoxy or other transparent material. A phosphor coating 84 comprising phosphor particles is applied on the inside surface of the lens 78 and on the top surface of the reflector 76. The top surface of the reflector 76, which may be thought of as the bottom of the package, is preferably first coated with a reflective layer 80, such as a high dielectric powder, such as alumina, titania, . . . etc. A preferred reflective material is Al2O3. The phosphor layer 84 is then placed over the reflective layer 80 on top of the reflector 76. The use of the reflective layer 80 serves to reflect any radiation 86 that penetrates the phosphor layer 84 on this surface.
Alternately, instead of coating the transparent lens 78 with a separate phosphor layer 84, the phosphor may instead be intimately dispersed within the material comprising the transparent hemisphere. The phosphor layer 84 over the reflective layer 80 on the reflector 76 is preferably relatively thick, i.e. >5 layers of powder, while the phosphor layer on the curved top of the hemisphere may be adjusted to achieve a desired color and to absorb all radiation incident on it. However, a proper approach to make the separate phosphor layer 84 is not disclosed.
Hence, a method for forming a uniform phosphor layer is desired. More specifically, a uniform phosphor layer formed in a curved shape is the key point to solve those problems mentioned above. The present invention uses electrophoretic deposition to form a uniform layer of phosphors. Meanwhile, the coated object is a pre-formed and transparent curved cup and free from the problem of lens shape. Therefore, the invention is a preferred solution for LED packaging lens to improve the uniformity of phosphor layer and lightening efficiency.